Recent outbreaks of severe acute respiratory syndrome (SARS) have been shown to be associated with the emergence of a novel virus belonging to the Coronaviridae family. While the outbreaks have been contained, the possible reemergence of the virus in the future prompts the urgent need for development of vaccines and antivirals, as well as for identification of the factors responsible for the high virulence, tissue tropism, and species-specificity of this virus. To date, the factors accounting for the unusually high pathogenicity of SARS virus in humans remain unknown. Based on our experience with other RNA viruses, we speculate that SARS viral infection stimulates activation of cellular pathways that would normally lead to induction of the type I interferon response and establishment of an antiviral state. We propose that in order to overcome this antiviral effect, the SARS virus, like other RNA viruses, may possess factors that would enable it to counteract the cellular innate immune responses. Using the assays available in our laboratory, we plan to screen for the presence of anti-interferon activity in SARS viral proteins, determine the signaling pathways at which this activity is exerted, and identify the specific points in each signaling cascade at which the inhibition occurs. Findings generated by these studies should help to expand our understanding of the interaction of RNA viruses with the elements of the host innate antiviral system, and may reveal possible targets of attenuation that could be used in the future for vaccine and antiviral drug development. Based on the findings of the first aim, the second part of our project will utilize the coronavirus reverse-genetics system to generate live SARS virus mutants knocked out/mutated for putative interferon antagonists. The pathogenicity, immunogenicity and protective effect of the generated virus mutants will be evaluated in mice, which were recently shown to be suitable models for the study of SARS virus infection. In addition, as an alternative vaccination approach, we will utilize recombinant Newcastle Disease Virus (rNDV) as a vector platform for development of SARS vaccines, rNDV vectors present an attractive strategy for the generation of SARS vaccines due to their safety and the lack of pre-existing immunity in humans. Using a reverse genetics system for rNDV that we have established in our laboratory, we will generate rNDV vectors expressing SARS virus S, N, E, and M proteins. Immunogenicity and protective effects of vaccination with the vectors will be evaluated in mice. Finally, the immunoprotective effect of rNDV vector and live attenuated virus vaccination will be tested in conjunction with other vectored SARS vaccines, such as alphavirus-based replicons generated by the other groups of our program project.